1. Area of the Art
The invention relates generally to silicon wafer manufacture, and more specifically to apparatus and methods for determining an epitaxial layer thickness and transition width.
2. Description of the Prior Art
Common and presently used epitaxial wafers in the micro-electronic industry often comprise highly doped substrates (low specific resistivity) and lower doped epitaxial layers (high specific resistivity.) Typical specifications of P/P+ and N/N+ wafers demand a well-defined layer thickness of the epitaxial layer, yet demand a certain transition width at the layer-substrate interface. The transition width is the depth at which measurable auto-doping occurs from the higher doped substance into the lower doped epitaxial layer.
Epitaxial wafers, such as P/P+ and N/N+ types used in the micro-electronic industry, display a significant difference in the doping concentration of the silicon substrate and the epitaxial layer. Such layers can be easily measured utilizing techniques designed to determine changes in doping concentrations or the conductivity type of the dopant as is in the case of P/N junctions.
Newer tendencies for present and future device applications, such as large wafer diameter processes for extremely high integration that is used for 64 Mb and 1 GB DRAM (Dynamic Random Access Memory) technologies, call for different specifications, however. Such epitaxial structures will require similar specific resistivities for substrates and layers, and are often referred to as P-/P- or N-/N- depending on the conductivity type. Future applications for state-of-the-art device manufacturing will most probably employ P-/P- or N-/N- epitaxial wafers. Consequently, such applications require the determination of epitaxial layer and substrate interactions.
Conventional measurements, utilizing methods sensitive to doping concentration changes to determine the epitaxial layer thickness or associated transition width, cannot be applied to the determination of the above discussed epitaxial layer and substrate interactions. Examples of typical and commonly used techniques to determine properties of epitaxial layers are the spreading resistance (SRP) method, the capacitance-voltage (C-V) method, the junction staining method and the Fourier Transform Infrared Spectrometry (FTIR) method. All these methods are designed to determine or correspond to changes in doping concentrations or the conductivity type of the dopant. They are not suitable to accurately determine epitaxial layer and substrate interactions if the changes in doping concentrations or the conductivity type are small.
Therefore, a need exists to develop a method which can be used to determine epitaxial layer thickness and transition width of a silicon wafer having similar specific resistivities for substrates and epitaxial layers.